ui: a signal's details sit directly under its name; its unit is user-changeable and saved with the session

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ui_app.hpp

@@ -1714,6 +1714,8 @@ void on_calibrate_apply(GtkButton*, gpointer user);
 /// Which of the calibration's two numbers a dialog edits; `Both` is the full dialog.
 enum class CalibField { Both, Gain, Offset };
 void open_calibration(App& app, GtkWindow* parent, std::size_t signal, CalibField field = CalibField::Both);
+/// The one-entry editor for a streamed signal's unit (an empty entry restores the declared one).
+void open_unit_editor(App& app, GtkWindow* parent, std::size_t signal);
 GdkContentProvider* on_drag_prepare(GtkDragSource*, double, double, gpointer user);
 int signal_color_index(const std::vector<SignalInfo>& signals, std::size_t signal);
 std::string color_hex(plot::Color c);